The present invention relates to a laser resonator for semiconductor lasers, as are used in particular in information technology and telecommunications and also in laser material processing.
In the field of information technology and telecommunications, semiconductor lasers are used in order to transmit signals for example in glass fibres. It is necessary thereby to focus the radiation to be coupled into a monomode fibre in diffraction-limited spots in order to achieve an efficient coupling. Energy-rich, diffraction-limited spots are striven for also in the field of laser material processing in order to make possible highly precise cutting and welding of the most varied materials.
There exist various possibilities in the state of the art for the production of monochromatic laser radiation with high energies by means of semiconductor lasers. These can be divided roughly into laser diode bars, laser diode stacks, broadband laser diodes or trapeze lasers.
Laser diode bars concern a number of individual laser diode structures which are located next to each other on a semiconductor laser substrate. The semiconductor substrate comprises a multilayer arrangement of different composition, the laser radiation being produced in one of the layers. This layer acts simultaneously for the produced laser light as layer waveguide which operates only one single vertical transverse mode via the layer thickness. In the layer plane, no spatial restriction of the waveguiding region is generally effected in the case of laser diode bars which leads to the produced laser light of a single emitter being strongly multimode in this direction. Between the individual emitters of such a laser diode bar there exists in general likewise no firm phase relationship of the produced laser beams so that the radiation of such a bar via the radiated face is spatially incoherent. A single bar is however able to radiate several 10W of optical power.
In order to achieve a further increase in the optical power, a plurality of laser diode bars is stacked one above the other so as to form so-called laser diode stacks, as a result of which optical total powers in the kW range are achieved. In addition, the concept of broadband single lasers exists furthermore, as high powers as possible being achieved to above the 10W range with band widths around 200 μm. The lateral radiation is however also extremely multimode in the case of broadband laser diodes.
In the case of the so-called trapeze lasers, a monomode radiation in a narrow band of a “master oscillator” is produced which is then coupled in an almost trapezoidal “power amplifier” range, which is optimised in its formation, and is amplified there. The hence achievable powers are around 2W. The available surface area of the laser band available for the amplification is thereby however not fully exploited for reasons of principle.
The development of Z-lasers started in F. Herrera, J. L. Verdegay, Genetic Algorithms and Soft Computing (Studies in Fuzziness and Soft Computing Vol. 8), Physica-Verlag Heidelberg, (1996), L. A. Vainstein, Diffrakzija v othkrytich resonatorach i otkrytych bolnovodach s ploskoimi zerkalami (Russian), Diffraction in open resonators and open waveguides with flat mirrors, Shurnal technitscheskoj fiziki (Journal for Technical Physics), Vol. 34, 193-204 (1964), is based on the angle selectivity during reflection in the limit range for total reflection. The power up till now has been 500 mW and the angle selectivity is approximately 2°. In addition to this selection angle there is a mode width of 35 μm which is expedient for diffraction-limited emission. An increase in the band width to be implemented for the purposes of increasing power to for example 200 μm permits no diffraction-limited emission in the case of a constant selection angle.
There exists in general in a laser resonator of a semiconductor laser, for example in a laser diode, a set of electromagnetic field distributions which are again completely reproduced during their propagation after a complete circulation in the resonator in intensity and phase up to a constant factor γ and hence exist in a stationary manner in the resonator. These discrete field distributions are called modes. In a laser, generally all the modes will oscillate during the laser operation, the circulating losses V of which, V=1−γ2 applying, being smaller than a value Vmax, designating thereby the so-called laser threshold, which is dependent predominantly upon the pump arrangement and in the active laser material.
If a laser oscillates only with the mode which has the lowest circulating losses then this is described as a monomode laser operation. This mode with the lowest circulating losses of the resonator is also called fundamental mode. During simultaneous oscillation of the laser with a plurality of modes this is described as multimode operation. The circulating losses of the individual laser modes are determined thereby by the resonator geometry, such as for example the optical elements in the resonator, by the resonator mirror, the apertures and geometric tolerances. By means of appropriate choice of this geometry, the circulating losses of the individual modes can be influenced specifically.
In semiconductor lasers, especially in broadband laser diodes and laser diode bars, the resonator geometry is given however by the width of the active, waveguiding layer and the resonator mirrors which are formed by the facets. The relatively large width of the active region in the case of semiconductor lasers in comparison to its length permits however a very large number of transverse modes to perform one resonator circulation with low losses so that semiconductor lasers in general operate in multimode operation without particular measures.
In all of the described variants of the semiconductor lasers, either their low power or the spatial incoherence of the produced laser radiation, which prevents effective focusing of the light in small spots, is consequently disadvantageous.
Methods are known from the literature for selection of individual longitudinal modes for frequency stabilisation of semiconductor lasers, for example by applying so-called Bragg gratings made of periodic refractive index distributions along the resonator axis on the laser chip. This is known for example from DE 43 22 163 A1. A selection of lateral laser modes is however not thereby effected since these Bragg gratings serve only for frequency stabilisation of the lasers.